The use of proteins as active agents has expanded in recent years due to several factors: improved techniques for identifying, isolating, purifying and/or recombinantly producing proteins; increased understanding of the roles of proteins in vivo due to the emergence of proteonomics; and improved formulations, delivery vehicles and approaches for chemically modifying proteins to enhance their pharmacokinetic or phamacodynamic properties. With respect to improved approaches for chemically modifying proteins, covalent attachment of a polymer such as poly(ethylene glycol) or PEG to a protein has been used to improve the circulating half-life, decrease immunogenicity, and/or reduce proteolytic degradation. This approach of covalently attaching PEG to a protein or other active agent is commonly referred to as PEGylation. Proteins for injection that are modified by covalent attachment of PEGs are typically modified by attachment of relatively high molecular weight PEG polymers that often range from about 5,000 to about 40,000 Daltons.
While modification of relatively large proteins for the purpose of improving their pharmaceutical utility is perhaps one of the most common applications of PEGylation, PEGylation has also been used, albeit to a limited degree, to improve the bioavailability and ease of formulation of small molecule drugs having poor aqueous solubilities. For instance, water-soluble polymers such as PEG have been covalently attached to artilinic acid to improve its aqueous solubility. See, for example, U.S. Pat. No. 6,461,603. Similarly, PEG has been covalently attached to triazine-based compounds such as trimelamol to improve their solubility in water and enhance their chemical stability. See, for example, International Patent Publication WO 02/043772. Covalent attachment of PEG to bisindolyl maleimides has been employed to improve poor bioavailability of such compounds due to low aqueous solubility. See, for example, International Patent Publication WO 03/037384. PEG chains attached to small molecule drugs for the purpose of increasing their aqueous solubility are typically of sizes ranging from about 500 Daltons to about 5000 Daltons, depending upon the molecular weight of the small molecule drug.
Active agents can be dosed by any of a number of administration routes including injection, oral, inhalation, nasal, and transdermal. One of the most preferred routes of administration, due to its ease, is oral administration. Oral administration, most common for small molecule drugs (i.e., non-protein-based drugs), is convenient and often results in greater patient compliance when compared to other routes of administration. Unfortunately, many small molecule drugs possess properties (e.g., low oral bioavailability) that render oral administration impractical. Often, the properties of small molecule drugs that are required for dissolution and selective diffusion through various biological membranes directly conflict with the properties required for optimal target affinity and administration. The primary biological membranes that restrict entrance of small molecule drugs into certain organs or tissues are membranes associated with certain physiological barriers, e.g., the blood-brain barrier, the blood-placental barrier, and the blood-testes barrier.
The blood-brain barrier protects the brain from most toxicants. Specialized cells called astrocytes possess many small branches, which form a barrier between the capillary endothelium and the neurons of the brain. Lipids in the astrocyte cell walls and very tight junctions between adjacent endothelial cells limit the passage of water-soluble molecules. Although the blood-brain barrier does allow for the passage of essential nutrients, the barrier is effective at eliminating the passage of some foreign substances and can decrease the rate at which other substances cross into brain tissue.
The placental barrier protects the developing and sensitive fetus from many toxicants that may be present in the maternal circulation. This barrier consists of several cell layers between the maternal and fetal circulatory vessels in the placenta. Lipids in the cell membranes limit the diffusion of water-soluble toxicants. Other substances such as nutrients, gases, and wastes of the developing fetus can, however, pass through the placental barrier. As in the case of the blood-brain barrier, the placental barrier is not totally impenetrable but effectively slows down the diffusion of many toxicants from the mother to the fetus in the art.
For many orally administered drugs, permeation across certain biological membranes such as the blood-brain barrier or the blood-placental barrier is highly undesirable and can result in serious side-effects such as neurotoxicity, insomnia, headache, confusion, nightmares or teratogenicity. These side effects, when severe, can be sufficient to halt the development of drugs exhibiting such undesirable brain or placental uptake.
U.S. Published Application No. 2003/0161791 A1, published Aug. 28, 2003, discloses water-soluble polymer conjugates of retinoic acid. The conjugates are prepared by covalent attachment of a water-soluble polymer such as polyethylene glycol to a retinoid such as retinoic acid. The conjugates are useful for inhalation therapy of conditions of the respiratory tract.
Thus, there is a need for new methods for effectively delivering drugs, and in particular small molecule drugs, to a patient while simultaneously reducing the adverse and often toxic side-effects of small molecule drugs. Specifically, there is a need for improved methods for delivering drugs that possess an optimal balance of good oral bioavailability, bioactivity, and pharmacokinetic profile. The present invention meets this and other needs.